Autofocus systems in cameras and camcorders for automatically focussing on a scene or object are known. Generally with such systems, a camera lens projects light representative of an image onto an image sensor such as a CCD or CMOS device. An autofocus (AF) module can retrieve data representative of a portion of the projected image, which can be processed by a microprocessor (CPU) in order to determine contrast information from the portion, for example. Using a suitable metric, the contrast information can provide a measure of the accuracy of focus. The CPU can activate an autofocus motor of the camera to move the camera lens for focusing, and the procedure can repeat until a subject falls within a desired level of focus.
U.S. Pat. No. 5,170,202 describes a method of autofocus based upon an image contrast metric similar to that described above.
Best focus is associated with the location of greatest image contrast as measured from a difference between brightest and darkest parts of an image.
In addition to contrast metrics, autofocus metrics can be based on various forms of high frequency image data such as gradients for example. In such cases it is necessary to overcome noise in the image data for better low light performance of the autofocus method.
In general, autofocus systems based on focus metrics have the disadvantage that the focus range must be searched at a granularity which is required to give accurate focus. Various techniques to speed up this search (such as stopping at a given point of focus when true focus is located) or locating the peak at a finer resolution than the search steps (such as using quadratic interpolation of the focus metric around a coarse peak), and methods to estimate true focus from as little as two steps are known. For example, U.S. Pat. No. 5,231,443 describes a rapid autofocus system. It is based on a depth from defocus method in which the difference in low frequency components of a pair of out of focus images is taken to depend on distance to the true focus. A ratio of the absolute power or absolute value of the low frequency components is used to define a metric which can be related to focus position.
This relationship only holds in general over a small region on either side of the true focus. At greater distances from true focus, only the very lowest frequency components of an image hold any useful information, and this is generally too little to reliably predict the true focus position.